This invention relates to visual acuity testing instruments, and more particularly, to visual acuity testers capable of displaying test characters with an improved appearance.
An important component of most eye examinations is a test of visual acuity, typically performed using a wall-mounted eye chart. Wall-mounted eye charts have rows of test characters of progressively decreasing size. Various test characters are commonly used, including Snellen letters, tumbling E's, tumbling C's, numbers, and other suitable patterns and test symbols. Typically, the person having his or her eyes examined (the patient) identifies as many of the test characters as possible. By determining the number of test characters of a given size that the patient is able to identify properly, the person performing the eye examination (the physician) is able to assess the patient's level of visual acuity.
Although it may be acceptable to use wall-mounted eye charts in some situations, wall-mounted eye charts are often cumbersome. Wall-mounted eye charts are designed to viewed only at a predetermined distance (known as the "lane length"). Unless the required lane length can be accommodated within the physician's office, the wall-mounted eye chart can not be used.
Physicians also examine the eyes of a variety of patients, so it is often desired to use different charts. For example, a chart made up of Snellen letters may be used to test an adult patient. When testing children, it may be preferable to use the tumbling "C" character set. Unfortunately, keeping an orderly set of wall-mounted eye charts is burdensome.
Another approach for visual acuity testing is to use a wall projector. Wall projectors are similar in design to slide projectors and allow various eye charts to be displayed on a wall for visual acuity testing. The size of the eye chart on the wall can be changed to accommodate different lane lengths by adjusting a variable magnification lens on the wall projector unit. However, making such adjustments requires careful calibration. In practice, physicians rarely change the magnification of the eye charts presented using wall projectors. In addition, providing a wide variety of charts with a wall projector can be burdensome. Because wall projectors are not capable of displaying randomly generated sets of test characters, patients sometimes memorize the sequence of test characters on the charts.
One way in which to overcome many of the drawbacks of traditional wall-mounted eye charts and wall projectors is to use an electronic display system. For example, commonly-assigned U.S. Pat. 5,121,981 by Waltuck et al., which issued Jun. 16, 1992, describes a display system in which a physician can electronically display a number of test characters at varying magnifications. However, the display system of the '981 patent is based on custom hardware, which is relatively expensive. Further, the visual quality of the test characters on the monitor in the '981 system is sometimes degraded by the staircase pixel effect known as "aliasing."
It is therefore an object of the present invention to provide an electronic display system for visual acuity testing that is economical and that can display test characters with improved visual appearance.